Method of injection molding granular materials



Sept. 26, 1944. w. R. TUCKER METHOD OF INJECTION MOLDING GRANULAR MATERIALS Original Filed Dec. 18, 1939 2 Sheets-Sheet 1 all I u I, ma h Sept. 26, 1944. w. R. TUCKER 2,359,013

METHOD OF INJECTION MOLDING GRANULAR MATERIALS Original Filed Dec. 18, 1939 2 Sheets-Sheet 2 [mhnkor Wmmm R Tuunm Hnorn -a Patented Sept. 26, 1944 2,859,018 METHOD OF INJECTION MOLDING GRANULAB MATERIALS Warren 3.. Tucker, Dayton, Ohio. neither to The Hydraulic Development Corp. Ina, Wilmington,

DeL, a corporation of Delaware Original application December 18, 1939, Serial No.

309,797. Divided 1941, Serial No. 888,471

(Cl. 1H5) comm.

This invention relates to a method of injection molding in which the temperature or the plastic material is more accurately controlled. The invention may be used in the injection molding of either thermosetting or thermoplastic resins, but is particularly adaptable to the molding of thermosetting materials.

Heretofore, it has been more or less common to heat the material in the injection cylinder by electric heating means, or the circulation of a heated fluid around the injection cylinder, whereby the granular non-fluent material is gradually heated up to the injectable state before it reaches the injection nozzle and in this fluent state is injected into the mold cavity. A popular practice has been to heat the injection cylinder with induction heat and to control the temperature of the cylinder wall by a suitable thermostat. This method is disadvantageous since it is impossible to impart an absolutely uniform temperature to the cylinder ,wall as this temperature fluctuates as much as 4 or 5. The result is that the temperature of the fluent material injected into the mold fluctuates several degrees. This is disadvantageous since the temperature of fluency of the commonly employed plastic materials is very critical.

My invention substantially eliminates fluctuation in the temperature of the injected material. This constancy of temperature is especially important in the case 0! thermosetting materials because. if they are heated above a certain critical temperature for any sustained length of time, an irreversible reaction takes place, reducing their capability of being rendered fluent and injectabie. My invention overcomes these disadvantages of the prior art practices by providing a uniform temperature in the injection cylinder. while the invention relates particularly to the maintenance of a uniform temperature in the injection cylinder of an injection molding machine. the principles thereof may, if desired, be applied in the maintenance of a uniform temperature in other portions of the injection molding machine, such as in a preheating chamber or in the mold halves.

The principal object of the invention is to hold the temperature in a portion of the injection molding machine at a uniform and constant level at which the material will be injected or will be rendered or maintained fluent.

Another object of the invention is to maintain the temperature or a thermosetting resin uniiormly at a point below the critical temperature of the resin, and to inject the resin into a mold and this application April 14,

in which it is raised to or above the critical setting point of the resin.

Still another object is to provide a progressively increasing temperature in a portion or the injection molding machine whereby the temperature in diilerent stages is completely uniform and is progressively increased as the plastic moves forward towards the mold.

Still other objects will more fully hereinafter appear.

This application is a division of my copending application, Serial No. 309,797, filed December 18, 1939 and now Patent No. 2,319,842.

In the accompanyins drawings:

Figure l is a vertical sectional view of one form of an apparatus embodying the principles of the present invention.

Figure 2 is a similar view of another form of apparatus embodyin the principles of the present invention and in which the temperature of the injection cylinder is progressively increased to a point below the critical setting point of the plastic, and is then injected into the mold in -which it is raised to a point above the critical setting point.

Figure 3 is a similar view of another form of apparatus embodyin the principles of the present invention and in which the fusion bath is electrically heated by a spiral immersion heater disposed in the bath. the bath being composed of a material which is a non-conductor of electricity when solid and a conductor of electricity when molten, electrical contacts being disposed in the bath and operative to maintain the bath at the melting point by maintaining a mixture of molten and solid bath material in intimate contact with one another; the electrical control mechanism is shown diagrammatically.

, Figure .4 is a similar view but showing another method of maintaining the bath material at the temperature at which change of phase from the solid state occurs; in this form oi. the invention the bath material is formed into two zones, the minor zone being disposed around the rear portion 0! the injection cylinder and being maintained at a temperature slightly above the melting point or temperature of change of phase, and the main body of the bath material being maintained at the melting point and near the point where it is all molten; the control portion of the bath at the rear thermostatically controls the application 0! heat to the whole body or fusion material or the like.

Figure 5 is a transverse vertical section throush an injection cylinder equipped with a fusion bath and with still other means for maintaining the bath at the melting point; in this view, the expanslon of the fusion material upon attaining the molten state is utilized to control the application of electrical energy to the heating element in the bath; the heating element is shown formed and disposed in such manner as to promote circulation of the bath material.

My invention involves the utilization of a bath oi normally solid material at a change of phase in a jacket surrounding the portion of the injection machine to be heated as an accurate means for controlling the temperature in that portion of the injection molding machine. As the fusion material for the bath, I may use metals having a melting point corresponding substantially to the temperature to be maintained in the portion of the machine in question. However. I may use other materials than metals which have a constant or fixed melting point and which are adapted to be held at the melting point for prolonged periods of time and which may be melted and solidified indefinitely. As an example of such other materials, I may use pure metals, chemical compounds, and eutectic mixtures, all of which melt and freeze at a constant temperature. For example, I may use sulphur, naphthalene, benzophenone, and certain salts which have the desired melting point such as, for example, potassium nitrate having a melting point of 337 C., a 45-55 mixture of sodium nitrate and potassium nitrate, which has a melting point of 218 C., or a 55-45 mixture of sodium nitrate and sodium nitrite which has a melting point of 221 C. As an example of still other materials which may be employed, I may under certain circumstances make use of the transition or cryohydric points of crystalline hydrates, where this transformation temperature is sufllciently high, this being another example of control materials having nonvariant points and comprising a solid and a liquid in admixture at the transition temperature. As examples of suitable alloys which may be employed, there may be mentioned an alloy consisting of equal weights of lead and tin, preferably with the addition of 0.1% of cadmium to improve the fluidity of the alloy, relatively low melting points alloys of bismuth, tin, lead and cadmium, such as those enumerated under the heading "Fusible metals in the book, Campbell's List of Alloys" 1930, page 52. Lead may be employed where a temperature of 327 is desired. An alloy of 30% lead and 70% tin maybe used where a temperature of 183 C. is employed. In addition to the foregoing alloys, the fusible alloys set forth on page 555 of the Handbook of Chemistry by Lange, 1934 or on page 488 of "Product Engineering volume (Nov. 1939), may be used. It will be understood that the fusible or like material is selected which has a melting or transformation temperature corresponding to that which it is desired to maintain in the in- Jectlon molding machine heating chamber.

' The fusible or like material is placed in a jacket surrounding the heating chamber. This jacket is provided with heating means of the usual type which has been heretofore employed directly around the heating chamber. This heating means may take the form of electrical resistance elements, or heating passageways through which a heated fluid such as hot oil is directed. Since the purpose of the present invention is to avoid the fluctuation in temperature of a heated fluid, it is usually preferred to employ an electric heating element at this point, the electric heating element being controlled by a suitable rheostat or other controlling means. If desired, a pyrometer which is adapted to indicate and control the temperature of the fusible or like material may be used. There may be provided thermostatic means operating in response to the temperature of the bath material or to the temperature in the heating chamber, whereby the electric heating element is automatically energized or de-energized to maintain the proper temperature and to compensate for the abstraction of heat from the heating chamber by the varied and non-uniform amounts of plastic material passed through the heating chamber.

By reason of the provision of a relatively large quantity of fusible or like material which is maintained at its melting point and which ordinarily has a high latent heat of fusion, the passage of material through the heating chamber does not ordinarily abstract suflicient heat from this body of fusible material to lower it below its melting point. In this way the temperature of the molten metal or the like does not drop and, correspondingly, the temperature of the wall of the injection cylinder and the temperature to which the plastic is heated, remains constant despite the abstraction of heat by the granular plastic. The large volume of melted metal or the like at its melting point acts as a. large reserve of heat from which heat may be abstracted intermittently without fluctuation in the actual temperature, advantage being taken of the phenomenon that the temperature of melting or of transition remains constant as long as the solid and the liquid phase are in lntermixture. In this way, the present invention enables a reduction in fluctuation of temperature in the heating chamber to a, negligible figure.

If desired, I may provide suitable means for preventing the electric heating element from heating the molten metal or the like above its fusion or transition point and for similarly preventing the molten metal from dropping below its melting point, thus insuring that a mixture of solid and molten metal is at all times present. Preferably, this mixture should be in such a state that the application or only a small amount of heat will convert it entirely to a molten form, thereby providing the maximum reserve of heat in the metal without allowing it to rise above the melting point. However, under certain circumstances, the metal may be allowed to rise above its melting point slightly, since the specific heat of the metal will usually be very small compared to the latent heat of fusion and, therefore, the passage of plastic through the heating chamber would soon lower the temperature to the melting point without doing undue damage. The means for insuring that the metal or the like remains at the fusion or transition point may take any suitable form. For example, it might comprise a. small portion of the fusible metal separated from the main body of the metal and adapted to more readily respond to the abstraction of heat by the passage of plastic through the heating chamber. A pyrometer or thermostat is connected to this separate portion in such manner that either the operator may manually or the thermostat may automatically apply the electric current to the heating element to an extent directly proportional to the drop in temperature of the separate portion of fusible metal, thereby preventing the temperature of the main body of fusible metal from droppin below the melting point. This means may take any other form of device for .plastic occurs causing fusible material at the stage where the solid and the fused material in intermixture and preferably at the point where almost all of the solid material has been converted to the molten form.

when using molten materials in the manner heretofore described the degree or temperature control over the plastic material passing through the heating chamber is very accurate, the injection temperature ofthe plastic being maintained constant at the melting point of the heating material. In the case of thermosetting resins the temperature of the material at the instant of injection is maintained below the critical temperature of the material and can be maintained sufiiciently accurate that the plastic material is injected while still in the granular state. The temperature of the material is then elevated while in the mold to bring the same to a fluent condition, at which time the chemical change of the setting thereof.

The accompanying drawings portray several methods of carrying out the invention. Referring first to Figure 1, reference numeral l designates the injection cylinder of an injection molding machine, in which reciprocates the injection plunger 2. The injection cylinder is provided with a screwed-in injection nozzle I and with a material spreader or torpedo l, and is adapted. to inject granular or fluent material into the mold cavity ii formed by the cooperating mold halves i and I, mold half I being stationary and mold half 6 being clamped thereto by means of the hydraulic motor 8. Surrounding the injection cylinder I is a jacket 9 of any suitable type adapted to retain the molten metal or the like and preferably constructed with a highly heat conductive cylindrical wall in order that the heat of the heating element may be readily transferred to the metal It or the like interposed between the jacket 9 and the exterior of the injection cylindo:- I. Surrounding the jacket 9 and in intimate contact therewith is an electrical heating element ll around which is disposed the heat inmaintaining a it comprises both sulating material IS. The electrical current is 45 supplied to the heating element II by the power lines II and IS, a rheostat I5 being interposed, if desired, in the lead ii. The temperature to which the metal I is heated and the application of electric current to indicated and controlled by the instrument II having the heat sensitive element It disposed at a suitable point in the bath material. The instrument I1 is an ordinary commercially available indicator and thermostatic control adapted to maintain the temperature of the metal It at the temperature to which instrument i1 is adjusted.

In Figure 1, the mold halves Ii and 1 are cored and are adapted to have a heated liquid 60 circulated therethrough by means of inlet pipe I! and outlet pipe 20. When operating with thermoplastic materials, the mold halves Ii and I may be maintained at a temperature below the setting point of the plastic. When operating with thermo-setting materials, they may be maintained at a temperature above the critical temperature of the resin and will thus cause the resin to assume its iniusible form after the expiration of the necessary interval.

In Figure 2 of the drawings, the general arrangement is similar to that 01' Fig. 1 except that a. series of separate zones for heating are employed, a iusible or like material being used in each zone which has a melting or phase conheating element ll may be and is version point corresponding to a temperature desired to be maintained in that zone. The zones Illa, Iiib and "lo are separated by partitions II. The individual heating elements Ha, I lb and I lc may be controlled manually, if desired, by the variable resistance lid, Ito and lie, electric current being supplied as before by lines It and It. It is preferable, however, to control the application of current to the heating elements by means or the control instruments lla, Ill: and IIc, as before. The materials Ilia, llib and Iiic for the baths are selected with a view to having their melting or phase conversion points correspond to the temperatures to be maintained in the zones of the injection cylinder, or the temperatures maintained in the baths may be selected with a view to obtaining the desired heat transfer. For example, it may be desired to maintain around zone A a relatively low temperature in order to prevent softening of the plastic in this zone, a somewhat higher temperature around zone B in order to begin the softening of the plastic, and a still higher temperature around the forwardmost zone C so as to render the plastic completely fluent, this last temperature being, however, below the critical setting point of the thermo-setting resin being injected. Or it might be desirable to use a high temperature around zone A in order to obtain very rapid transfer of heat to the plastic material and to use a relatively low temperature around zones B and C in order to convert the material to fluent form but to prevent burning of the material, the temperature in the final zone C being optionally just under the critical setting point of the resin to maintain the resin fluent or to maintain the temperature of the resin just below the point of fluency to permit the same to be injected into the mold in a granular state. When injecting thermo-setting materials with the arrangement of Figure 2, the mold halves 8 and I are heated to a temperature above the critical temperature of the resin by means of electric heating elements 22 or by the circulation of a heated fluid as in Figure 1.

Instead of surrounding the fusion bath with the electric heating element as shown in Figs. 1 and 2, a rod-like commercially available heating element may be spirally wound around the injection cylinder I within the material of the bath. An example of such a heater is t e ordinary commercially available Calrod" immersion heater manufactured by the General Electric Company. By using such an immersion heater, particularly in conjunction with the use of a metal as the bath material, a perfect contact is obtained between the heater and the bath, whereby heat losses are minimized and rapid heating is assured.

It will be seen from the foregoing that the use of a bath Ill eliminates the necessity for a circulating pump for circulating the heated material heretofore employed and prevents local overheating of portions of the injection cylinder. In addition, it enables the maintenance of an exactly predetermined temperature of the bath and a corresponding temperature of the plastic material. Where the materials employed for the bath are good heat conductors, such as molten metal or fused salts, a better contact is obtained with the wall of the injection cylinder and with the surface of the heating element, thereby resulting in faster and more uniform heating of the plastic. Instead oi using conducted heat for heating the bath, I may use induced heat, at-

ranging the bath to act as the low tension winding oi a transformer and effecting heating ductively with eddy currents of high frequency. For. the higher temperature ranges, I may use molten metals or fused salts, whereas for the lower temperature ranges, I may use the transformation temperatures of crystalline hydrates, such as, for example, copper sulphate at 110 C., barium chloride at 100 C., barium hydroxide at 78 0., sodium sulphate at 32 0., manganese chloride at 58 0., trisodium phosphate at 73 0., sodium bromide at 51 0., sodium carbonate at 35 C. and sodium thiosulphate at 48 C. It will be understood that the plastic may not attain the temperature of the jacket, but may stay below the temperature of the jacket, the temperature difference being determined by the particular plastic, the particular speed of operation, and the particular die employed, the temperature of the Jacket being predetermined by experiments conducted before the actual injection process, and once having been determined, being maintained at the temperature of change of phase as long as the particular conditions are utilized.

Instead of maintaining the temperature at the change of phase in the case of fusible materials, I may, under certain circumstances carry it above the fusing point and use an ordinary thermostatic control to maintain it at approximately the desired temperature, For example, in the case of an alloy melting at 90 C., I may operate with this alloy maintained at a temperature of substantially 100 0., or higher, if desired. In such a case, I prefer to use a highly heat conductive material for the bath, such as metal or iused salt in order to attain the advantages of better heat conduction thereby as compared with the hot oil which has been heretofore commonly employed.

Referring now to the modification shown in Figure 3, the general construction is the same as that heretofore described, except that a spiral rodlike heating element 23 is wound around the injection cylinder I and is within the material of the bath, and this heating element is controlled by the state of the material of the bath. The bath material 24 is carried within a suitable jacket 25 surrounded by heat insulation 28. The material of the bath is such that it will not conduct electricity when solid but will conduct electricity upon the change of phase, such as upon becoming molten. For example, the bath material may be a fused salt such as the salts enumerated above, the salt being selected which has the desired melting point and when molten and when solid. Instead of using a fused bath, I may, under certain circumstances. employ a crystalline hydrate at the temperature at which it loses water. In such case, the material below the transition temperatur will not conduct electricity but above the transition temperature will conduct electricity. In order to insure that the bath material 2 is maintained at the melting point or the like, I provide a pair of electrical contact 21' which are adapted to be electrically connected when the bath material therebetween becomes molten and to thereby allow electrical current to flow through the secondary of transformer 29 and through the winding 28 of solenoid 30, thereby opening switch 3| and opening the electrical circuit to heating element II which is normally established by reason of a spring normally holding switch Ii in closed position, the incoming power lines being designoted as 32. The contacts 11 are well removed electrical conducting properties from the heater element 23 so that a large portion of bath material 24 is in molten form before the circuit is established across contacts 21. In this way. a large reserve of latent heat of fusion is available for heating the plastic in the injection cylinder. When switch ii is opened, the supply of electricity to heater element 21 is out oh and solidification oi the bath material in the region oi contacts 21 may occur by reason or the loss of heat from the bath by radiation or by ex traction thereot by the plastic material. By using the apparatus 01' Fig. 3. the necessity for using an expensive thermostat is eliminated, the bath itself being employed to control the application of electrical energy and thereby to maintain the bath at the temperature of conversion and at a point where the major portion of the bath is in the converted state.

In Figure 4 there is shown an apparatus wherein the same general arrangement is employed as in Figure 3, except that the necessity for employing a material which is an electrical conductor when converted but a non-conductor when in the solid state, is eliminated, and wherein a different method or maintaining the bath at the temperature of conversion is employ In this embodiment, a molten metal bath maybe employed as well as any of the materials which would be suitable for use in the apparatus of Figure 3. The bath is divided into two portions, a main portion 33 and a relatively small control portion 34 which is disposed around a rear portion or iniection cylinder l, these portions 33 and 14 being separated by a partition 35. A heater element 23 is employed as in Fig. 3, with the major porof the spiral heating element in control section 34, whereas the spacing of the turns in the main portion 33 is such that only one turn is used in a portion of bath 33 corresponding in length and siz to control bath I4. A thermostat 38 is provided having a sensitive element 41 disposed in the control bath 34 at a considerable distance from the heating element 23, and this thermostat 36 is adapted to actuate a snap switch I8 disposed in one of the incoming power lines 82 and to thereby cut oil the application of heat to heater 23 when a predetermined temperature in the control bath 34 has been reached. Thermostat I! need not be extremely sensitive and, therefore, need not be unduly expensive, it being satisfactory if thermostat 38 is responsive to temperature changes oi the order of 3 or 4 F. Thermostat 36 is adjusted so as to maintain the temperature in control portion 34 at 3 or 4 above the melting point of the bath material. For example, if thermostat I6 is accurate to within 3 plus or minus, it may be set to operate at a temperature or say 4 above the melting point oi the bath material. Due to the fact that thermostat II is set to operate at a temperature slightly above the melting pointand to the fact that the proportionate heat applied to the control portion 34 is greater than that to the main portion 33, the result will be that the main bath 33 will be maintained at the melting temperature and at a point where it is practically all in the molten or converted form. The maintenance oi control bath 84 at a higher temperature is not harmful because the plastic at the rear of the in- Jectlon cylinder is granular and is not capable of being injured by the excessive temperature at this point, although the subiection or the fluent piastiotosuchatsmperaturemightoonvortit totheinnnibleiorm. Fromtheicregoing,itwill beseenthatthearrangcmentotl'ialprovidooa simpleandine pemivemeansoimaintainingthe mainbodyoithebothattheconversiontomm terial It is heated by electrical heating element Ii which is generally oi. the type described in co-pending application I. B. Lawyer, Serial No. 231,637, flied September 26, 1938, and is adapted to cause convection circulation of the material of the bath and to thereby eliminate local overheating. As the material of the bath becomes molten, it urges outwardly a plunger 42 which is normally spring d inwardly and which operates within a cylinder 43 mounted in the wall oi the jacket. when plunger 4! moves outwardly due to the expansion of the metal or the like, it opens a snap switch 4 disposed in one of the power lines 3! leading to the heater 4|, cutting oil the supply 0! electrical current and allowing the bath 40 to remain at its molten temperature and preventing it from being heated above the temperature 01' melting. As heat is abstracted from bath ill, the bath shrinks and plunger 42 is spring-pressed inwardly; causing snap switch 44 to close, thus again energizing heater 4| and causing the temperature of the bath It to rise. By disposing the plunger at the top 0! the bath and at a point well removed from heater element ll, it is insured that a large portion oi the bath N is in molten form before the electric current is cut oil to heater ll. Thus, by taking advantage of the property of the bath material 0! expanding upon change of state to the molten form, I amenabled to maintain the temperature of the bath at the melting point and near that point where it is all molten but without raising its temperature above the melting point, without the use or an expensive thermostat.

In Figures 3 and 4, an expansion space it is provided above the bath to allow for expansion thereof upon heating, and in the disclosure oi. Fig. 3 the expansion oi the heating material can make or break electric circuit through the contacts 21, upon a rise or tail of the level of the material.

From the foregoing description, it will be seen that I have devised an inexpensive and eilective method and apparatus for heating a plastic to a constant temperature using the latent heat of a heating material, this method and apparatus being particularly applicable to the injection molding oi thermo-setting materials which are exceedingly susceptible to small increases in temperature over a critical temperature, although the invention may equally be applied to thermoplastic materials where constant conditions of operation are desired. These results are accomplished in a preferred form by the maintenance of a bath in direct heat conducting relationship with the injection cylinder, this bath being maintained at the constant temperature at which it undergoes a change of phase from the solid state. I consider that invention resides not only in the method and apparatus for injection molding, but also in the particular controlling means oirig.8,i'orexample. Itwillbeseentha have devised mechanism which not tains the bath material at the temperature oi change of phase, but which maintains it at a point where the change oi phase is almost entirely complete. so that the large reserve or latent heat attendant upon extensive change of phase is available at a constant temperature to heat the plastic.

Theprocessmaybe carriedoutbyheating the granular or powdered theme-setting plastic in the injection cylinder to a suitable low temperature at which it retains its granular form, and injecting it under pressure into the mold while it retains its granular form. the temperature to whichthemoldisheatedandthepressureto which the granular material is subjected by the injection plunger after it is in the mold. being suillciently high to cause the material to flow into a homogeneous form and to subsequently set. In the case of granular thermoplastic materlals the mold is heated sumciently and the injection pressure exerted on the granular plastic in the mold is such as to cause the granular material in the mold to flow to take the shape or the article after which the mold cools or is cooled to harden the article.

The granular thermo-setting plastic may be preheated in the injection cylinder to a temperature at which it is suillciently soft to be injected (say 60 to C.) under the usual injection pressures and then injected into the mold while it retains its unreacted state, the mold being heated as before to a temperature above the critical temperature oi the resin and supplied with suflicient heat at the temperature to set the article.

I wish it to be understood that I intend to include as within my invention such modifications and adaptations thereof as may be necessary to make it suitable for varying conditions and uses and as tall within the scope of the appended claims.

Having thus iully described my invention, what I claim as new and desire to secure by Letters Patent is:

l. The process of injection molding which comprises heating granular plastic in an injection chamber to a relatively low temperature to heat the same but to retain its granular term, and injecting it in granular form into a hot mold to prevent any substantial reduction in temperature in the transfer from the injection chamber to the mold, the mold being heated sumciently and the injection pressure applied on the material in the mold being sumcient to cause the granular injected material to rapidly plasticize and flow into a homogeneous article taking the form of the mold.

2. The process of injection molding which comprises heating granular unreacted thermosetting plastic in an injection chamber to a relatively low temperature to retain its granular unreacted iorm, injecting it in granular form into a. hot mold, the mold being heated sufllciently and the injection pressure applied on the material in the mold being sufllcient to cause the granular iu jected material to plasticize and flow into a homogeneous article taking the form of the mold, and to elevate the temperature oi. the granular material within the mold to reaction temperature for permanently setting the same.

3. The process of injection molding which comprises heating a granular plastic in an injection chamber to a controlled low temperature at which it retains its granular form, of injecting the heated material while in granular form and at the low temperature into a hot mold that is at a temperature substantially above the temperature of the plastic material to rapidly elevate the temperature of the plastic to that of the mold whereby to plasticlze the same, and of causing the plasticized material to set.

4. The process of injection molding which comprises elevating the temperature oi a granular plastic in an injection chamber to a relatively low temperature at which it retains its granular form by subjecting the plastic to the action of latent heat derived directly from a body of normally solid material maintained at a temperature at which the material undergoes a change of phase from the solid state, and injecting the plastic material in granular form into a mold heated to a temperature substantially above the temperature 01' the granular plastic, the mold being heated sumciently and the injection pressure applied on the material in the mold being suilicient to cause the granular injected material to plasticize and flow into a homogeneous article taking the form 01' the mold.

5. The process of injection molding which comprises heating granular unreacted thermo-setting plastic in an iniection chamber to a relatively low temperature at which it retains its granular form, controlling the temperature of the granular plastic to retain its granular form by subjecting the plastic to the action of latent heat derived directly from a body of normally solid material maintained at a temperature at which the material undergoes a change of phase from the solid state, and injecting the plastic in granular form into a mold heated to a temperature substantially above the temperature of the granular plastic, the mold being heated suiiiciently and the injection pressure applied on the material in the mold being sufflcient to cause the granular injected material to plasticlze and flow into a homogenecus article taking the form of the mold and to subsequently react and set to the solid state.

6. The process of injection molding which comprises elevating the temperature ot a granular plastic in a plurality of independent stages to a maximum temperature at which it retains its granular form, individually controlling the temperature of each of the stages, and of injecting the plastic into a mold in granular form at the temperature oi the final stage.

7. The process of injection molding which comprises sequentially subjecting granular plastic to the action of latent heat derived directly from a plurality of normally solid bodies of material maintained at diil'erent temperatures and at which the several independent bodies undergo a change in phase from the solid state to elevate the temperature of the granular plastic in a plurality of stages to a maximum temperature at which it retains its granular form, individually controlling the temperature of each of the stages, and of injecting the plastic into a mold in granular form and at the temperature of the final stage.

8. The process of injection molding which comprises heating a zone through which plastic material passes to a temperature substantially equal to the physical transformation temperature of a heating material when passing from the solid to the liquid state and surrounding the zone, of passing granular plastic material through the zone to elevate the temperature thereof to that of the zone without changing the granular condition of the material, of electing the plastic from the zone into a mold in a granular state, and of controlling the temperature of the heating material to maintain the same at said transition temperature thereof to maintain the temperature of the granular plastic constant just prior to 40 ejection.

WARREN R. TUCKER. 

